1. Field of the Invention
The present invention relates to a plant controlling system and a plant controlling method in which a process value of a plant is controlled by operating a plurality of operation targets.
2. Description of the Related Art
FIG. 8 is an illustrated diagram showing a general plant provided with operating valves 101 and 102 serving as control valves for adjusting process values, such as pressure and flow rate.
To improve the controllability (controlling function) at low flow rates, two types of operating valves having different capacities, such as a low-capacity operating valve 101 (operating valve A) and a high-capacity operating valve 102 (operating valve B), are arranged in parallel. In general, a plant controlling system 100 controls the low-capacity operating valve 101 and the high-capacity operating valve 102 based on process values detected by a process value detector 103 in such a manner that the low-capacity operating valve 101 is used at low flow rates, and the high-capacity operating valve 102 is brought into use when the flow rate increases.
From the viewpoint of cost, a parallel arrangement of two 50%-capacity operating valves can be used instead of a single 100%-capacity operating valve, or a parallel arrangement of three or more 50%-capacity operating valves including one used as a backup during maintenance can be used.
For automatic control of the plural operating valves A and B arranged in parallel, which are operation targets, the conventional plant controlling system 100 uses the split control method, in which a manipulated variable MV output from positional-type PID controlling unit 104 is supplied to function transforming units 105 and 106 provided for the operating valves A and B to separately operate each valve as shown in FIG. 9.
In many cases, the conventional plant controlling system 100 further includes manual operating unit 107 that operates the plural operating valves A and B arranged in parallel in a manual mode by switching the operation mode from an automatic mode to the manual mode. In that case, typically, the plural operating valves A and B are regarded as one control target, and one shared manual operating unit 107 is provided upstream from the function transforming units 105 and 106 provided for the operating valves A and B.
More specifically, in the plant controlling system 100, a control set value SV output from set value setting unit 108 and a process value PV detected by the process value detector 103 are input to signal comparing unit 109, and the signal comparing unit 109 determines the deviation between the control set value SV and the process value PV and outputs a control deviation “e”. The positional-type PID controlling unit 104 outputs a manipulated variable MV for each of the operating valves A and B according to the control deviation “e”.
The manual operating unit 107 receives the manipulated variable from the positional-type PID controlling unit 104 and outputs the manipulated variable MV received from the positional-type PID controlling unit 104 in the case where the operator selects the automatic mode or outputs a manipulated variable MV set by manual operation in the case where the operator selects the manual mode.
The manipulated variable MV output from the manual operating unit 107 is input to the function transforming units 105 and 106 provided for the operating valves A and B. The function transforming units 105 and 106 transform the manipulated variable MV according to transform functions 110A and 110B shown in FIG. 10, for example, and the resulting manipulated variables MV-A and MV-B are output to the operating valves A and B, respectively.
The transform functions 110A and 110B shown in FIG. 10 are examples in the case where the operating valve A is the low-capacity operating valve 101 and the operating valve B is the high-capacity operating valve 102. The transform function 110A is a function that transforms a manipulated variable MV of 0 to 30% input from the manual operating unit 107 into a manipulated variable MV-A of 0 to 100% to be output. The transform function 110B is a function that transforms a manipulated variable MV of 30 to 100% input from the manual operating unit 107 into a manipulated variable MV-B of 0 to 100% to be output. That is, when the input manipulated variable MV ranges from 0 to 30%, the operating valve A is opened or closed in the range from 0 to 100%, and when the input manipulated variable MV ranges from 30 to 100%, the operating valve B is opened or closed in the range from 0 to 100%.
Furthermore, since the plant controlling system 100 includes the positional-type PID controlling unit 104, tracking is carried out by outputting the manipulated variable MV output from the manual operating unit 107 to the positional-type PID controlling unit 104 as a tracking signal T when the operating valves A and B are switched between the automatic mode and the manual mode, for example, switched from the automatic mode to the manual mode. For example, when the manual operating unit 107 is switched from the manual mode to the automatic mode, the tracking allows the positional-type PID controlling unit 104 to output the manipulated variable MV determined by calculation (integration) beginning with the value of the manipulated variable MV immediately before the switching, and therefore, an abrupt change of the manipulated variable MV can be prevented.
If the separate manual operating unit 107 is provided downstream side from the function transforming units 105 and 106, the manipulated variables MV output from the manual operating unit 107 have different values, and therefore, the tracking based on the manipulated variable MV cannot be achieved. Therefore, the system having one shared manual operating unit 107, such as the plant controlling system 100, is commonly used.
A plant controlling system (a process controlling system) described in Patent Document 1 (Japanese Patent Laid-Open No. 9-190201) includes positional-type PID controlling unit including a velocity-type PID controlling unit and an integrator and carries out tracking by the integrator performing integration of a manipulated variable deviation output from the velocity-type PID controlling unit, function transforming unit (a manipulated variable distributor) transforming (distributing) the manipulated variable calculated by the integrator for each operating valve (operation end) and outputting the resulting value to each operating valve, and calculating a tracking signal from a process quantity controlled by each operating valve and outputting the tracking signal to the integrator.
The conventional plant controlling system 100 shown in FIGS. 8 to 10 has only one shared manual operating unit 107 for the plural operating valves A and B, and therefore, the operating valves A and B cannot be separately manually operated. This does not pose a problem in the normal manual operation. However, for example, if a mechanical problem occurs in any of the plural operating valves A and B and affects automatic control, it is preferred that only the operating valve, to which the problem is caused, is switched to the manual mode and excluded from the targets of automatic control, and the remaining operating valve remains under automatic control.
In addition, the plant controlling system described in the Patent Document 1 has the positional-type PID controlling unit, and therefore, has to carry out logically complicated tracking in order to prevent an abrupt change of the manipulated variable output to an operating valve when the operating valve is switched from the manual mode to the automatic mode, for example.
In addition, in some cases, a flow rate sensor for detecting a process value has to be additionally provided in order to carry out the tracking. As a result, the cost of the plant controlling system substantially increases.